This section introduces aspects that may help facilitate a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
3GPP LTE (Long Term Evolution) uses the Physical Downlink Control Channel (PDCCH) in the downlink to issue scheduling decisions for uplink and downlink transmissions. For uplink transmissions, the information sent on the PDCCH informs which mobiles are allowed to send packet data transmissions on the physical uplink shared channel (PUSCH), and for downlink transmissions the PDCCH informs particular mobiles that data will be sent to them on the physical downlink shared channel (PDSCH). Proper reception of the PDCCH is crucial for proper operation of the LTE air interface, which relies exclusively on the shared channel concept. That is, because users must share a common channel for their transmissions, it is crucial that users receive information regarding when they are allowed to transmit in the uplink or when they will be receiving information in the downlink.
The PDCCH was designed in the LTE standard to work properly in a reuse-1 environment; that is, it was designed to be able to properly reach mobiles located at the edge of the cell where the signal to interference plus noise ratio (SINR) may be quite low, say −5 dB. A PDCCH transmission is done using a set of control channel elements (CCEs), and the LTE standard allows the aggregation of 1, 2, 4, or 8 CCEs which allows lower coding rates for the information transmitted on the PDCCH while consuming a larger amount of bandwidth to transmit the message. The highest aggregation level allowed is aggregation level 8, which allows approximately 10*log 10(8)=9 dB lower SINR to be experienced on the PDCCH compared to the case of no aggregation being used.
However, new scenarios are now being considered to enhance overall LTE system performance through the introduction of small cells which are located near areas of high traffic density within a macro-cellular network, and which transmit at a low power level, say 16 dB below a normal macro cell base station, as illustrated in diagram 100 of FIG. 1. Such a deployment of cells is referred to as a heterogeneous network. Cell association biases are introduced in these heterogeneous networks to allow the coverage of the small cells to be extended to allow overall improved system performance; however, this can create a very poor interference condition in the downlink.
For example, if the cell selection bias used in this heterogeneous network is modified such that a mobile connects to the cell for which it measures the smallest path loss (i.e., closest radio distance), then a mobile, which is connected to the low power small cell and is located at the border of the small cell coverage area and the macro cell coverage area, will receive downlink transmissions from the high-power macro cell which is at a much higher power level (equal to the difference in transmit power between the macro cell and the small cell, potentially 16 dB stronger) than the downlink transmissions from the small cell, as illustrated in diagram 200 of FIG. 2. Because the mobile is connected to the small cell, the high power macro cell is considered interference; this means the cell edge SINR may be nearly 16 dB lower than experienced in existing homogenous networks.
This creates a problem for PDCCH reception on the downlink, as the existing aggregation levels in the standard are not designed to handle such a low SINR condition at the edge of the cell. Therefore, the need exists for new techniques that improve the operation of the crucial PDCCH in these high interference conditions.
Specific embodiments of the present invention are disclosed below with reference to FIGS. 1-5. Both the description and the illustrations have been drafted with the intent to enhance understanding. For example, the dimensions of some of the figure elements may be exaggerated relative to other elements, and well-known elements that are beneficial or even necessary to a commercially successful implementation may not be depicted so that a less obstructed and a more clear presentation of embodiments may be achieved.
Simplicity and clarity in both illustration and description are sought to effectively enable a person of skill in the art to make, use, and best practice the present invention in view of what is already known in the art. One of skill in the art will appreciate that various modifications and changes may be made to the specific embodiments described below without departing from the spirit and scope of the present invention. Thus, the specification and drawings are to be regarded as illustrative and exemplary rather than restrictive or all-encompassing, and all such modifications to the specific embodiments described below are intended to be included within the scope of the present invention.